Jerry's incredibly keen to follow the example of the other boys on the beach and swim through a narrow tunnel that runs through a large rock. Jerry clearly wants to emulate the boys, to convince himself that he's every bit as good as they are when it comes to swimming.
To that end, Jerry heads off on his own to practice diving. As part of his practice, he spends a lot of time holding his breath under water. This will be essential if he's going to follow the example of the boys on the beach and swim through the tunnel.
On the whole, things seem to be going swimmingly for Jerry. He's able to lie on the bottom of the sea, holding a rock without too much difficulty. However, at nighttime, he's afflicted with nosebleeds, a direct consequence of holding his breath under water for extended periods of time.
Jerry's mother senses that her son has been overdoing his training somewhat, and when his nose bleeds a second time, she insists that he come with her to the beach the following day. Although this deprives Jerry of some much-needed training, he subsequently discovers that a day's rest has actually improved his count by ten.